1. Technical Field of the Invention
The present invention relates to an antenna apparatus, and more particularly, to a small planar antenna apparatus used as a global positioning system (GPS) antenna.
2. Related Art
As known in this technical field, a GPS (global positioning system) measures the position of an object using a satellite. The GPS receives radio waves (GPS signals) from four or more satellites among 24 satellites that orbit the earth, measures the positional relationship between a moving object and the satellites and a time error therebetween using the received radio waves, and accurately calculates the position and altitude of the moving object on the map using triangulation.
In recent years, the GPS has come into widespread use for car navigation systems for detecting the position of a traveling vehicle. The car navigation system includes a GPS antenna for receiving GPS signals, a processing unit that processes the GPS signals received through the GPS antenna to detect the current position of a vehicle, and a display unit that displays the position detected by the processing unit on the map.
Meanwhile, in recent years, with the development of small communication apparatuses (for example, a GPS car navigation apparatus, a portable navigation apparatus, and a satellite receiver), such as mobile communication apparatuses, antenna apparatuses having a small size and a high performance have been demanded.
Among the antenna apparatuses, a planar antenna apparatus (for example, a circularly polarized wave patch antenna) has the advantages of a small thickness, a small size, and easy integration with a semiconductor circuit. Therefore, the planar antenna apparatus is widely used as an antenna for a small communication apparatus.
The planar antenna apparatus including a circularly polarized wave antenna element and a circuit board having a low noise amplifier (LNA) on a rear surface thereof has been proposed (for example, see Patent Documents 1, 3 and 4). The circularly polarized wave antenna element is composed of a so-called patch antenna element. The circularly polarized wave antenna element includes a dielectric substrate that is formed of a high dielectric material such as ceramic. A radiating element is provided on the front surface of the dielectric substrate, and a ground pattern is formed on the rear surface of the dielectric substrate. A pinhole is formed in the dielectric substrate so as to pass through the dielectric substrate from the front surface to the rear surface. A feeding pin for connecting the radiating element and the circuit board is inserted into the pinhole. In the planar antenna apparatus having the above-mentioned structure, the dielectric substrate formed of a high dielectric material makes it possible to ensure the capacitance of the antenna, and thus it is possible to lower a resonance frequency and reduce the size of the planar antenna apparatus.
As another planar antenna apparatus, a metal plate patch antenna having an antenna element (a radiating conductor plate) that is formed of a metal plate has been proposed. The metal plate patch antenna can be manufactured at a lower cost than a general planar antenna apparatus having a radiating conductor layer (radiating element) that is patterned on a dielectric substrate.
The metal plate patch antenna having the following structure has been proposed: it includes a ground conductor, a dielectric substrate (a circuit board) that has a plurality of solder lands and is provided on the ground conductor, a radiating conductor plate that is provided above the dielectric substrate (the circuit board) at a predetermined distance therefrom, and a plurality of legs that extend from the radiating conductor plate toward the dielectric substrate (the circuit board); and the plurality of legs are soldered to the corresponding solder lands to support the radiating conductor plate (for example, see Patent Document 2). In the planar antenna apparatus (the metal plate patch antenna) having the above-mentioned structure, the plurality of legs extending from the radiating conductor plate toward the dielectric substrate (the circuit board) are soldered to the solder lands, and the solder lands are opposite to the ground conductor with the dielectric substrate (the circuit board) interposed therebetween. Therefore, capacitors are formed by capacitances between the solder lands and the ground conductor. As a result, the resonance frequency is lowered, and thus it is possible to reduce the size of the radiating conductor plate.
[Patent Document 1] JP-A-2001-339232
[Patent Document 2] JP-A-2005-143027
[Patent Document 3] JP-A-2001-339233
[Patent Document 4] JP-A-2001-339234
However, the high dielectric material, such as ceramic, is heavy and expensive. Therefore, as described in Patent Documents 1, 3 and 4, when the high dielectric material, such as ceramic, is mounted on a small planar antenna apparatus, the overall weight of the antenna apparatus increases, and the manufacturing costs thereof also increase.
Meanwhile, in the planar antenna apparatus (the metal plate patch antenna) disclosed in Patent Document 2, the size of the radiating conductor plate is reduced, but the capacitance value changes due to a variation in the amount of solder used and a variation in soldering area. As a result, the planar antenna apparatus (the metal plate patch antenna) disclosed in Patent Document 2 has problems in that the antenna resonance frequency varies and stable frequency characteristics are not obtained.
In addition, electrodes (lands) may be formed on the front surface (main surface) and the rear surface of the circuit board in order to increase the capacitance. However, in this structure, it is necessary to increase the areas of the electrodes (lands) in order to increase the capacitance value and improve the effect of shortening a wavelength, which results in an increase in the size of the substrate.